1. Field of the Invention
The present invention relates generally to an engine for use in an outboard motor, and more particularly to a small engine with a balancer mechanism.
2. Description of the Relevant Art
When pistons reciprocally move in an engine, the engine violently vibrates as a whole under inertial forces generated by reciprocating movement of the piston. Usually, a plurality of crank webs or counterweights are mounted on the crankshaft in order to cancel out the inertial forces. Upon rotation of the crankshaft caused by reciprocating movement of the piston, other inertial forces are produced in a direction normal to the direction in which the pistons move.
Japanese Utility Model Publication No. 52-32802 discloses a counterbalance mechanism for suppressing engine vibration arising from the latter inertial forces. The disclosed counterbalance mechanism generally includes two balancer shafts disposed one on each side of the crankshaft parallel thereto and operatively coupled to the crankshaft by gears, chains, or the like, for being rotated in opposite directions at the same speed as that of the crankshaft, and balancers mounted on the balancer shafts. The crank webs counterbalance half of the reciprocating mass including crankshaft-connected members such as linearly movable pistons and connecting rods, and each of the balancers counterbalances one quarter of that reciprocating mass. The balancers and the crank webs are oriented in the same direction at the top and bottom dead centers of the pistons. This counterbalance mechanism is effective in eliminating inertial forces produced in the direction of movement of the pistons and also inertial forces generated in the direction normal to the direction of movement of the pistons by the crank webs and the balancers, and in preventing a couple of forces from being produced. The disclosed counterbalance mechanism can therefore reduce engine vibration effectively.
Since the balancers are disposed on the diametrically opposite sides of the crankshaft, they have to be sufficiently spaced from the crankshaft so as to be held out of physical interference with the crank webs. As a result, the engine is large in size as a whole. Moreover, attachment of engine components and accessories such as a carburetor and an engine starter is highly complex, i.e., not enough space is available for easy installation of these engine components and accessories. The width of the engine may be increased dependent on how these engine components and accessories are located.